Gibson GA-8T 120hz.Hum & No Tremolo

[Helmholtz]

Lift one end of each 0.02µ coupling cap and see if this changes the hum or tube currents.

[misterc57]

Lift one end of each 0.02µ coupling cap and see if this changes the hum or tube currents.

So I lifted one end of each .02 coupling cap. No difference in currents. No hum.

I noticed the 1.5K resistors following the coupling caps feeding the grid are actually 10K (brown, black, orange), not as stated in the schematic. Is this an issue?

In the pics the green wire coming in from the bottom is from the "depth" pot.

Thank you! MarkO

[Helmholtz]

So I lifted one end of each .02 coupling cap. No hum.

So the hum is not coming from the power stage.

I noticed the 1.5K resistors following the coupling caps feeding the grid are actually 10K (brown, black, orange), not as stated in the schematic. Is this an issue?

No issue at all.

[g1]

Does the volume control affect the level of the hum?
If not, the hum source must be between the volume control and those .02 coupling caps from PI tube. (V1B and V2A stages)

[SoulFetish]

if the only thing connected to pin 3 of the rectifier tube is the two center tap wires coming from the transformer and the floating black wire, than your power supply ground isn't connected to the transformer center tap and will cause major hum (this is what I suspected in my first post). take a clip lead and connect pin 3 to the terminal where you grounded the filter caps and see if that helps. If t does, then solder a wire connecting them.

[misterc57]

Does the volume control affect the level of the hum?
If not, the hum source must be between the volume control and those .02 coupling caps from PI tube. (V1B and V2A stages)

Volume control does increase hum. No hum at volume zero, hum gets way loud past 5. and louder at 10.

[misterc57]

if the only thing connected to pin 3 of the rectifier tube is the two center tap wires coming from the transformer and the floating black wire, than your power supply ground isn't connected to the transformer center tap and will cause major hum (this is what I suspected in my first post). take a clip lead and connect pin 3 to the terminal where you grounded the filter caps and see if that helps. If t does, then solder a wire connecting them.

pin 3 is at ground, as is the floating black wire. I think pin 3 is connect to ground from a white wire connected to all the pot grounds. I will take another look and try connecting the floating black wire to my common ground point.

[misterc57]

Connecting rectifier pin 3 floating black wire to my new ground (where all the filter caps are grounded) did not change anything.

Not sure what to try next. Perhaps move all grounds to the same point?

I have hum at pin 6 of V1 which gets louder as I dial up the volume pot.

[Helmholtz]

Are you sure you have 120Hz not 60Hz hum?

Can you scope the output?

[misterc57]

Output, volume set to 5

[g1]

That is some high frequency noise riding on the hum. Slow the timebase way down.
24.43uSec period is around 41Khz.

[Helmholtz]

That is some high frequency noise riding on the hum.

I'd say the ripple hum will be riding on the HF.
This is a full-tilt (35Vrms) 41kHz oscillation.
Very strange with an amp that doesn't use global NFB.

[misterc57]

If I slow down the time (I tried going to around 50 ms) I do not get a readable picture. The time shown was the only way to show two full waves.

Still learning and I am lost of the HF oscillation parlance. Is this 120 hz hum? My phone listening app says that it is.

What to try next?

Thank you

[Helmholtz]

Set time base to 5ms/div, select trigger source "line" and adjust trigger level to get a stable display.

What happens if you set the trem pot to zero?

Edit: Please ignore what I wrote before. Do not disconnect the wiper. Rather the wiper should be at ground potential.

[Enzo]

Here is my simple trick. Touch the scope probe tip with your finger - that will put 60Hz on the screen. It may not look pretty. Now set the sweep so that 60Hz is one full cycle (or two if you like). Now do not change the sweep speed, but now probe your hum signal. If it also has the one full cycle across the screen (or two if you prefer), then the hum is also 60Hz. If however the scope now shows twice as many cycles, then it is 120Hz. Takes far less time to do than to describe it.